29 research outputs found
Outflow or galactic wind: The fate of ionized gas in the halos of dwarf galaxies
Context: H\alpha images of star bursting irregular galaxies reveal a large
amount of extended ionized gas structures, in some cases at kpc-distance away
from any place of current star forming activity. A kinematic analysis of
especially the faint structures in the halo of dwarf galaxies allows insights
into the properties and the origin of this gas component. This is important for
the chemical evolution of galaxies, the enrichment of the intergalactic medium,
and for the understanding of the formation of galaxies in the early universe.
Aims: We want to investigate whether the ionized gas detected in two
irregular dwarf galaxies (NGC 2366 and NGC 4861) stays gravitationally bound to
the host galaxy or can escape from it by becoming a freely flowing wind.
Methods: Very deep H\alpha images of NGC 2366 and NGC 4861 were obtained to
detect and catalog both small and large scale ionized gas structures down to
very low surface brightnesses. Subsequently, high-resolution long-slit echelle
spectroscopy of the H\alpha line was performed for a detailed kinematic
analysis of the most prominent filaments and shells. To calculate the escape
velocity of both galaxies and to compare it with the derived expansion
velocities of the detected filaments and shells, we used dark matter halo
models.
Results: We detected a huge amount of both small scale (up to a few hundred
pc) and large scale (about 1-2 kpc of diameter or length) ionized gas
structures on our H\alpha images. Many of the fainter ones are new detections.
The echelle spectra reveal outflows and expanding bubbles/shells with
velocities between 20 and 110 km/s. Several of these structures are in
accordance with filaments in the H\alpha images. A comparison with the escape
velocities of the galaxies derived from the NFW dark matter halo model shows
that all gas features stay gravitationally bound.Comment: 15 pages, 13 figures, accepted for publication in A&
A kinematic study of the irregular dwarf galaxy NGC 2366 using HI and Halpha observations
Abridged. Context. The metal content of dwarf galaxies and the metal
enrichment of the intergalactic medium both suggest that mass loss from
galaxies is a significant factor for the chemical evolution history of
galaxies, in particular of dwarf galaxies. However, no clear evidence of a
blow-away in local dwarf galaxies has been found so far.
Aims. We therefore performed a detailed kinematic analysis of the neutral and
ionised gas in the nearby star-forming irregular dwarf galaxy NGC 2366 in order
to make predictions about the fate of the gas and to get a more complete
picture of this galaxy.
Methods. A deep Halpha image and Fabry-Perot interferometric data of NGC 2366
were obtained. They were complemented by HI synthesis data from the THINGS
survey. We searched for line-splitting both in Halpha and HI by performing a
Gaussian decomposition. To get an idea whether the expansion velocities are
high enough for a gas blow-away, we used the pseudo-isothermal halo model,
which gives us realistic values for the escape velocities of NGC 2366. The good
data quality also allowed us to discuss some peculiarities of the morphology
and the dynamics in NGC 2366.
Results. A large red-shifted outflow north west of the giant extragalactic
HII region with an expansion velocity of up to 50 km/s is found in Halpha, but
not in HI. Additionally, a blue-shifted component north of the giant
extragalactic HII region was detected both in Halpha and HI with an expansion
velocity of up to 30 km/s. A comparison with the escape velocities of NGC 2366
reveals that the gas does not have enough kinetic energy to leave the
gravitational potential.Comment: 15 pages, 14 figures, accepted for publication by A&
The intriguing HI gas in NGC 5253: an infall of a diffuse, low-metallicity HI cloud?
(Abridged) We present new, deep HI line and 20-cm radio continuum data of the
very puzzling blue compact dwarf galaxy NGC 5253, obtained with the ATCA as
part of the `Local Volume HI Survey' (LVHIS). Our low-resolution HI maps show
the disturbed HI morphology that NGC 5253 possesses, including tails, plumes
and detached HI clouds. The high-resolution map reveals an HI plume at the SE
and an HI structure at the NW that surrounds an Ha shell. We confirm that the
kinematics of the neutral gas are highly perturbed and do not follow a rotation
pattern. We discuss the outflow and infall scenarios to explain such disturbed
kinematics, analyze the environment in which it resides, and compare it
properties with those observed in similar star-forming dwarf galaxies. The
radio-continuum emission of NGC 5253 is resolved and associated with the
intense star-forming region at the center of the galaxy. We complete the
analysis using multiwavelength data extracted from the literature. We estimate
the SFR using this multiwavelength approach. NGC 5253 does not satisfy the
Schmidt-Kennicutt law of star-formation, has a very low HI mass-to-light ratio
when comparing with its stellar mass, and seems to be slightly metal-deficient
in comparison with starbursts of similar baryonic mass. Taking into account all
available multiwavelength data, we conclude that NGC 5253 is probably
experiencing the infall of a diffuse, low-metallicity HI cloud along the minor
axis of the galaxy, which is comprising the ISM and triggering the powerful
starburst. The tidally disturbed material observed at the east and north of the
galaxy is a consequence of this interaction, which probably started more than
100 Myr ago. The origin of this HI cloud may be related with a strong
interaction between NGC 5253 and the late-type spiral galaxy M 83 in the past.Comment: 19 pages, 12 figures, accepted for publication in MNRA
Three-dimensional modeling of the HI kinematics of NGC 2915
The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a
large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI
synthesis observations of NGC 2915 have been obtained with the Australia
Telescope Compact Array. These observations provide evidence of extremely
complex HI kinematics within the immediate vicinity of the galaxy's
star-forming core. We identify and quantify double-peaked HI line profiles near
the centre of the galaxy and show that the HI energetics can be accounted for
by the mechanical energy output of the central high-mass stellar population
within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI
data cube are generated and compared to the observations to test various
physical scenarios associated with the high-mass star-forming core of NGC 2915.
Purely circular HI kinematics are ruled out together with the possibility of a
high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities
of ~ 30 km/s are required to describe the central-most HI kinematics of the
system. Our results lend themselves to the simple physical scenario in which
the young stellar core of the galaxy expels the gas outwards from the centre of
the disc, thereby creating a central HI under-density. These kinematics should
be thought of as being linked to a central HI outflow rather than a large-scale
galactic blow-out or wind.Comment: 11 pages, 6 figures, accepted for publication in MNRA
On the nature of faint Low Surface Brightness galaxies in the Coma cluster
This project is the continuation of our study of faint Low Surface Brightness
Galaxies (fLSBs) in one of the densest nearby galaxy regions known, the Coma
cluster. Our goal is to improve our understanding of the nature of these
objects by comparing the broad band spectral energy distribution with
population synthesis models. The data were obtained with the MEGACAM and CFH12K
cameras at the CFHT. We used the resulting photometry in 5 broad band filters
(u*, B, V, R, and I), that included new u*-band data, to fit spectral models.
With these spectral fits we inferred a cluster membership criterium, as well as
the ages, dust extinctions, and photometric types of these fLSBs. We show that
about half of the Coma cluster fLSBs have a spectral energy distribution well
represented in our template library while the other half present a flux deficit
at ultraviolet wavelengths. Among the well represented, ~80% are probably part
of the Coma cluster based on their spectral energy distribution. They are
relatively young (younger than 2.3 Gyrs for 90% of the sample) non-starburst
objects. The later their type, the younger fLSBs are. A significant part of the
fLSBs are quite dusty objects. fLSBs are low stellar mass objects (the later
their type the less massive they are), with stellar masses comparable to
globular clusters for the faintest ones. Their characteristics are correlated
with infall directions, confirming the disruptive origin for part of them.Comment: Accepted for publication in A&A, 10 pages, 10 figure
A low H I column density filament in NGC 2403 : signature of interaction or accretion
Date of acceptance: 12/07/2014Observed H i accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion occurs in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To constrain the presence and properties of such flows, we present deep H i observations obtained with the NRAO Green Bank Telescope of an area measuring 4° × 4° around NGC 2403. These observations, with a 5σ detection limit of 2.4 × 1018 cm-2 over a 20 km s-1 linewidth, reveal a low column density, extended cloud outside the main H i disk, about 17′ (~ 16 kpc or ~ 2 R25) to the NW of the center of the galaxy. The total H i mass of the cloud is 6.3 × 106 M⊙, or 0.15 percent of the total H i mass of NGC 2403. The cloud is associated with an 8 kpc anomalous-velocity H i filament in the inner disk, that was previously observed in deep VLA observations. We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigboring dwarf galaxyPeer reviewe
Magnetic fields in Local Group dwarf irregulars
We wish to clarify whether strong magnetic fields can be effectively
generated in typically low-mass dwarf galaxies and to assess the role of dwarf
galaxies in the magnetization of the Universe. We performed a search for radio
emission and magnetic fields in an unbiased sample of 12 Local Group (LG)
irregular and dwarf irregular galaxies with the 100m Effelsberg telescope at
2.64 and 4.85GHz. Magnetic fields in LG dwarfs are three times weaker than in
the normal spirals (<4.2+-1.8muG). The production of total magnetic fields
appears to be regulated mainly by the star-formation surface density, with the
power-law exponent of 0.30+-0.04, or by the gas surface density (with the
exponent 0.47+-0.09). In addition, we find systematically stronger fields in
objects of higher global star-formation rate. The dwarf galaxies follow a
similar far-infrared relationship (with a slope of 0.91+-0.08) to that
determined for high surface brightness spiral galaxies. The magnetic field
strength in dwarf galaxies does not correlate with their maximum rotational
velocity, indicating a small-scale rather than a large-scale dynamo process. If
magnetization of the Universe by galactic outflows is coeval with its metal
enrichment, we show that more massive objects (such as Lyman Break Galaxies)
can efficiently magnetize the intergalactic medium with a magnetic field
strength of about 0.8nG out to a distance of 160-530kpc at redshifts 5-3,
respectively. Several times weaker fields and shorter magnetization distances
are expected from primordial dwarf galaxies. We also predict that most
star-forming local dwarfs might have magnetized their surroundings up to about
0.1muG within about 5kpc distance. Strong magnetic fields (>6muG) are observed
only in dwarfs of extreme characteristics while typical LG dwarfs are
unsuitable objects for the efficient supply of magnetic fields to the
intergalactic medium.Comment: Published in Astronomy and Astrophysics, 15 pages, 11 figures, minor
changes to version 1, Fig. 6 changed, discussion of interactions enlarge
Non-circular motions and the cusp-core discrepancy in dwarf galaxies
Context. The cusp-core discrepancy is one of the major problems in astrophysics. It
results from comparing the observed mass distribution of galaxies with the
predictions of cold dark matter simulations. The latter predict a cuspy
density profile in the inner parts of galaxies, whereas observations of
dwarf and low surface brightness galaxies show a constant-density core.
Aims. We want to determine the shape of the dark matter potential in the nuclear
regions of a sample of six nearby irregular dwarf galaxies.
Methods. In order to quantify the amount of non-circular motions that could
potentially affect a mass decomposition, we first perform a harmonic
decomposition of the H